Organic and polymer light-emitting diodes (LEDs) have been the subject of intensive investigation in recent years because of their potential applications as emissive elements for flat panel displays and white light sources for general lighting. In general, LEDs comprise multilayer configurations having hole-transporting, emissive, and electron-transporting layers to balance the injection and transport of both holes and electrons. Holes and electrons are injected from opposing electrodes into the light-emitting layer and recombine to form excitons. Light emits as a result of the emissive decay of the excitons.
For LEDs based on small molecules, it is rather straightforward to adopt this multi-layer strategy via layer-by-layer vacuum deposition. For polymer-based LEDs, films can be formed by spin-coating techniques via solution processing.
There are several advantages of polymer-based LEDs made by spin-coating techniques. Spin-coating techniques can significantly reduce the production cost and allow for large area coating. In addition, using spin-coated hole-transporting polymers instead of the vapor-deposited small-molecule materials can potentially increase the glass transition temperature along with improving the thermal and morphological stability of the devices.
A challenge in fabricating multilayer devices by spin coating is that the first spin-coated layer can be adversely affected by being re-dissolved by the solvent used in spin coating the second layer. As a result, the first coated layer must have resistance against solvents used in forming the second layer.
Hole-transporting layers (HTLs) play an important role in fabricating high efficiency multilayer polymer light-emitting diodes (PLEDs). HTLs allow enhanced hole injection from an indium tin oxide (ITO) anode into a light-emitting layer (EML) and results in balanced charge injection/transport and better device performance. Hole-transporting materials (HTMs) are often the first layer applied to an anode followed by sequential layer-by-layer fabrication of a LED. Therefore, for use as a HTL in multilayer PLEDs, the hole-transporting material needs to have solvent resistance to allow spin-coating of a second overlying layer.
A variety of approaches have been reported to overcome interfacial mixing that occurs during the spin-coating process: thermal or photochemical crosslinking before applying the second layer; formation of a self-assembly layer on ITO; and utilization of solubility difference between polar and non-polar solvents. However, many of these approaches involve complicated and low efficiency polymer synthesis and produce devices with inconsistent and irreproducible quality. Therefore, there is a need for high purity and low-molecular-weight HTMs that can directly undergo in-situ crosslinking and simultaneously generate solvent-proof networks.
Recently, the efficiency of PLEDs has been significantly increased by incorporating phosphorescent dopants in polymer hosts to harvest both singlet and triplet excitons. In this case, both singlet and triplet excitons formed in the host can be transferred to the phosphorescent dopant by Förster and Dexter energy transfer processes, allowing the devices to reach high internal quantum efficiency. In blue- or green-emitting electrophosphorescent PLEDs that include high energy phosphors, large bandgap host materials with triplet energy higher than that of the phosphorescent emitter are needed in order to avoid back energy transfer from the triplet dopants to the host. Due to high HOMO energy level of these large gap hosts (usually greater than about −5.8 eV), it is difficult to achieve efficient hole injection from ITO into the EML if only a single HTL is employed in the device configuration. To alleviate these problems, multiple HTLs with stepped electronic profiles to provide cascade hole-injection and transport are needed to achieve improved hole injection and charge confinement, and to attain improved efficiency in PLED devices.
The same concept can be applied to the quantum dot (QD) OLEDs as well, where the hole injection is extremely inefficient due to the high HOMO energy levels of the QDs (usually greater than about −6.0 eV).
A challenge exists to realize a multi-HTL structure in PLEDs because the multilayer structure preferably includes HTLs having solvent resistance and compatibility. Recently, cascade hole injection has been demonstrated through the layer-by-layer deposition of a series of photocrosslinkable triarylamine derivatives with oxetane functionality. Other polymers, such as poly(p-phenylenevinylene) (PPV), poly(N-vinylcarbazole) (PVK), poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butyl phenyl)imino)-1,4-phenylene) (TFB), and the ethanol-soluble PVK salt, PVK-SO3Li, have also been mentioned as suitable materials for functioning as the second HTL between poly(3,4-ethylenedioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS) and EML. However, the polymers noted above either produce reactive by-products during conversion process (e.g., PPV) or bear metal ions (e.g., PVK-SO3Li) that are detrimental to the long-term stability of the devices. In addition to these problems, none of above-mentioned materials possess significantly high triplet energies to confine blue-excitons, further limiting their use in electrophosphorescent devices. Therefore, there is a need for a multi-HTL structure in PLEDs in which the hole-transporting materials possess high triplet energies, as well as solvent resistance and compatibility.
Despite advances in the development of LED devices and hole-transporting materials, a need exists for compounds with solvent resistance and compatibility, for hole-transporting structures with improved hole injection and charge confinement, and for LED devices with long-term stability and high efficiency. The present invention seeks to fulfill these needs and provides further related advantages.